Environmental Requirements
The caper bush requires a semiarid climate. Mean annual temperatures in areas under cultivation are over 14°C and rainfall varies from 200 mm/year in Spain to 460 in Pantelleria and 680 in Salina. In Pantelleria, it rains only 35 mm from May through August, and 84 mm in Salina. A rainy spring and a hot dry summer are considered advantageous. This drought-tolerant perennial plant has favourable influence on the environment and it is used for landscaping and reducing erosion along highways, steep rocky slopes, sand dunes or fragile semiarid ecosystems.
A harvest duration of at least three months is necessary for profitability. Intense daylight and a long growing period are necessary to secure high yields. The caper bush can withstand temperatures over 40°C in summer, but it is sensitive to frost during its vegetative period. The potential exposure of caper hydraulic architecture to cavitations has recently been proposed as an explanation for its susceptibility to freezing conditions. On the other side, a caper bush is able to survive low temperatures in the form of stump, as it happens in the foothills of the Alps. Caper plants are found even 3,500 m above sea level in Ladakh, though they are usually grown at lower altitudes. Some Italian and Argentine plantings can withstand strong winds without problems, due to caper bush decumbent architecture and the coriaceous consistency of the leaves in some populations.
The caper bush is a rupicolous species. It is widespread on rocky areas and is grown on different soil associations, including alfisols, regosols and lithosols. In different Himalayan locations, C. spinosa tolerates both silty clay and sandy, rocky or gravelly surface soils, with less than 1% organic matter. It grows on bare rocks, crevices, cracks and sand dunes in Pakistan, in dry calcareous escarpments of the Adriatic region, in dry coastal ecosystems of Egypt, Libya and Tunisia, in transitional zones between the littoral salt marsh and the coastal deserts of the Asian Red Sea coast, in the rocky arid bottoms of the Jordan valley, in calcareous sandstone cliffs at Ramat Aviv, Israel, and in central west and northwest coastal dunes of Australia. It grows spontaneously in wall joints of antique Roman fortresses, on the Wailing Wall, and on the ramparts of the castle of Santa Bárbara (Alicante, Spain). Clinging caper plants are dominant on the medieval limestone-made ramparts of Alcudia and the bastions of Palma (Majorca, Spain). This aggressive pioneering has brought about serious problems for the protection of monuments.
The caper bush has developed a series of mechanisms that reduce the impact of high radiation levels, high daily temperature and insufficient soil water during its growing period.
The caper bush has a curious reaction to sudden increases in humidity; it forms wart-like pock marks across the leaf surface. This is apparently harmless, as the plant quickly adjusts to the new conditions and produces unaffected leaves.
It also shows characteristics of a plant adapted to poor soils. This shrub has a high root/shoot ratio and the presence of mycorrhizae serves to maximize the uptake of minerals in poor soils. Different nitrogen-fixing bacterial strains have been isolated from the caper bush rhizosphere, playing a role in maintaining high reserves of that growth-limiting element.
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